Patterned doping of semiconductor substrates using photosensitive monolayers

ABSTRACT

A semiconductor device and a method of fabricating a semiconductor device are disclosed. Embodiments of the invention use a photosensitive self-assembled monolayer to pattern the surface of a substrate into hydrophilic and hydrophobic regions, and an aqueous (or alcohol) solution of a dopant compound is deposited on the substrate surface. The dopant compound only adheres on the hydrophilic regions. After deposition, the substrate is coated with a very thin layer of oxide to cap the compounds, and the substrate is annealed at high temperatures to diffuse the dopant atoms into the silicon and to activate the dopant. In one embodiment, the method comprises providing a semiconductor substrate including an oxide surface, patterning said surface into hydrophobic and hydrophilic regions, depositing a compound including a dopant on the substrate, wherein the dopant adheres to the hydrophilic region, and diffusing the dopant into the oxide surface of the substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to semiconductors, and morespecifically, to patterned doping of semiconductor substrates usingphotosensitive monolayers. Embodiments of the invention, even morespecifically, relate to such patterned doping of extremely thinsemiconductor layers.

2. Background Art

In order to be able to make integrated circuits (ICs), such as memory,logic, photovoltaic, and other devices, of higher integration densitythan currently feasible, one has to find ways to further downscale thedimensions of semiconductors, such as complementary metal oxidesemiconductors (CMOS) and photovoltaic devices. Scaling achievescompactness and improves operating performance in devices by shrinkingthe overall dimensions and operating voltages of the device whilemaintaining the device's electrical properties. Additionally, alldimensions of the device must be scaled simultaneously in order tooptimize the electrical performance of the device.

With conventional semiconductor scaling reaching fundamental limits, thesemiconductor industry is looking at more unconventional and newapproaches that will facilitate continued device performanceimprovements. As a result, attention has been given to usingsemiconductors with ultra or extremely thin silicon layers where thesilicon or “device” layer has a thickness of from about seven nm andabout ten nm.

Ultra thin semiconductor devices have very substantial advantages,however they also present difficult challenges. For instance, thesedevices can experience threshold-voltage and subthreshold slopefluctuation because of silicon thickness variations across the wafer.For example, a common silicon-on-insulator (SOI) device may have asilicon layer thickness of from 4-8 nm, with a variation in thisthickness of 1 or more nm across the wafer.

Also, it has been determined that when conventional procedures are usedto implant dopants into semiconductor layers that have a thickness of 10nm or less, the ion implantation amorphizes the semiconductor layer.Recrystallizing the amorphous semiconductor layer is difficult, becauseof the limited amount of crystal seed layer that is available insemiconductor layers having a thickness of less than 10 nm that havebeen ion implanted into an amorphous crystal structure. The presence ofan amorphous semiconductor material in a semiconductor device results inthe semiconductor device having a high external resistance. Further, theresistance of the semiconductor device is increased by defects in thesemiconductor layer that are produced by ion implantation. The ionimplantation may also damage the gate dielectric.

As a result, conventional ion implantation procedures appear to beunsuitable for doping ultra thin semiconductor devices.

BRIEF SUMMARY

Embodiments of the invention provide a semiconductor device and a methodof fabricating a semiconductor device. Embodiments of the invention usea photosensitive self-assembled monolayer to pattern the surface of asubstrate into hydrophilic and hydrophobic regions, and an aqueous (oralcohol) solution of the dopant compound (e.g., boric acid, organicboronic acid, phosphonic acids, etc) is deposited by spin coating ordoctor blading upon which these materials will only deposit on thehydrophilic regions. After deposition, the substrate is then coated witha very thin layer of oxide (e.g. aluminum oxide or hafnium oxide) byatomic layer deposition to cap the compounds and the substrate isannealed at high temperatures to diffuse the dopant atoms into thesilicon and to activate the dopant.

In one embodiment, the method comprises providing a semiconductorsubstrate including an oxide surface, patterning said surface intohydrophobic and hydrophilic regions, depositing a compound including adopant on the substrate, wherein the dopant adheres to the hydrophilicregion, and diffusing the dopant into the oxide surface of thesubstrate.

The dopant compound may, for example, be an aqueous or alcohol solution.In one embodiment, the method further comprises capping the dopantcompound on the substrate. This capping, for example, may includeforming an oxide layer over the oxide surface of the substrate. In anembodiment, the diffusing includes annealing the substrate to diffusethe dopant into the oxide surface of the substrate; and in anembodiment, this annealing is done after the capping the dopantcompound.

In one embodiment, the patterning includes applying a monolayer of aphotosensitive material to said oxide surface; and exposing themonolayer to radiation to form the monolayer into a pattern ofhydrophobic and hydrophilic regions. The dopant compound adheres to thehydrophilic regions, and does not adhere to hydrophobic regions of themonolayer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows a semiconductor structure having a monolayer of aphotosensitive material.

FIG. 2 depicts the attachments of different types of photosensitivematerial to the semiconductor structure.

FIG. 3 illustrates a patterned beam of radiation applied to thestructure of FIG. 1.

FIG. 4 shows the patterned monolayer formed by the applied radiation.

FIG. 5 shows a dopant solution deposited on the patterned monolayer ofFIG. 4.

FIG. 6 illustrates a capping oxide layer formed on the dopedsemiconductor structure of FIG. 5.

DETAILED DESCRIPTION

Detailed embodiments of the present invention are disclosed herein;however, it is to be understood that the disclosed embodiments aremerely illustrative of the invention that may be embodied in variousforms. In addition, each of the examples given in connection with thevarious embodiments of the invention is intended to be illustrative, andnot restrictive. Further, the figures are not necessarily to scale, andsome features may be exaggerated to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

In one embodiment, the present invention relates to a semiconductordevice and to a method of fabricating a semiconductor device. Generally,in one embodiment, the invention uses a photosensitive self-assembledmonolayer to pattern the surface of a semiconductor substrate intohydrophilic and hydrophobic regions, and an aqueous (or alcohol)solution of a dopant compound (e.g. boric acid, organic boronic acid,phosphonic acids, etc.) is deposited on the semiconductor surface by,for example, spin coating or doctor blading. These deposited materialswill only attach on the hydrophilic regions. After deposition, thesubstrate is then coated with a very thin layer of oxide (e.g. aluminumoxide or hafnium oxide) by atomic layer deposition, vacuum evaporationor alternatively by solution deposition of spin-on glass or spin-onhanium oxide followed by annealing to cap the compounds, and then thesubstrate is annealed at high temperatures to diffuse the dopant atomsinto the silicon and to activate the dopant.

FIG. 1 illustrates an exemplary structure 10 comprising a substrate 12and a monolayer of a photosensitive material 14. The substrate 12 maycomprise an insulator material, a semiconductor material, or a metallicmaterial. For example, substrate 12 may be Si, although other suitablesemiconductor materials may be used. For example, SiC, SiGe, SiGeC, Sialloys, Ge, Ge alloys, GaAs, InAs, and InP, as well as other III/V andII/VI compound semiconductors, may be used.

The top surface 16 of the substrates contacts the monolayer of thephotosensitive material 14, and this surface 16 comprises an oxidesurface, which may contain a metal oxide or a semiconductor oxide. Themetal oxide includes at least one elemental metal. The semiconductoroxide, in one embodiment, is a compound of a semiconductor material andoxygen, and this semiconductor material may be, for example, silicon,germanium, carbon, or an alloy thereof.

The monolayer of the photosensitive material 14 is attached to the oxidesurface 16 by covalent bonds. The photosensitive material comprisesthree groups: a first end group 20, a photosensitive group 22, and asecond end group 24. The first end group 20 forms the covalent bondswith the oxide surface 16, which, as mentioned above, comprises eitheran oxide surface or a semiconductor material and oxygen. With referenceto FIGS. 1 and 2, the first end group 20 may, as examples, comprise acarboxylic acid, a phosphonic acid or, a hydroxamic acid.

The photosensitive group 22 is linked to the first end group 20, and thephotosensitive group 22 dissociates upon exposure to ultravioletradiation. Also with reference to FIGS. 1 and 2, the photosensitivegroup 22 may comprise a nitribenzyl moiety having a photosensitive bondsuch as a CO—O—CH₂ bond or a CH₂—NH—CO bond.

The second end group 24 is linked to the photosensitive group 22 andprovides hydrophobicity to the photosensitive material. The second endgroup 24 may comprise one of hydrogen, fully or partially fluorinatedalkyl, fully or partially fluorinated alkoxy, or fully or partiallyfluorinated alkylthio, and the total number of carbon atoms in thisalkyl, alkoxy, or alkylthio may, in embodiments of the invention, befrom 1 to 20.

The covalent bonds between the first end group 20 in each molecule ofthe photosensitive compound 14 and the oxide surface 16 enablesself-assembly of the photosensitive material on the oxide surface 16 asa monolayer. This is because the covalent bond, represented at 26, isformed only between the acidic end group of the photosensitive materialand the oxide surface, and more specifically, this bond is formedbetween a plurality of oxygen atoms of the end group of thephotosensitive material, and that oxide surface 16. The molecules of thephotosensitive material do not provide atoms available for covalentbonding except for those that are bonded to the oxide surface.

As illustrated in FIG. 3, the structure 10 is subjected to a patternedbeam of ultraviolet radiation. A conventional lithography tool and aconventional lithographic mask may be employed to generate the patternedbeam of ultraviolet radiation. Application of a photoresist onto thestructure 10 is not necessary, though, since the monolayer 14 of thephotosensitive material reacts with the patterned beam of ultravioletradiation.

With reference again to FIGS. 2 and 3, the photons of the patterned beamof ultraviolet radiation react with the photosensitive group 22 todissociate the photosensitive group 22 within the exposed region of themonolayer of the photosensitive material 14. As the photosensitive group22 dissociates by the ultraviolet radiation, the second end group 24having hydrophobicity is removed along with some atoms of thephotosensitive group 22. The remaining portion 32 of the photosensitivegroup 22 forms a derivative of the photosensitive group having anexposed hydrophilic functionality. After removal of the cleaved portioncontaining the second end group 24, for example, in an aqueous oraqueous alcohol solution, the exposed portion of the monolayer 14containing the exposed photosensitive material has a hydrophilicsurface.

With reference to FIG. 4, the monolayer is thus patterned and includesat least one first region 42 and at least one second region 44. Thefirst region 42 comprises the photosensitive material, and the secondregion 44 has a lithographic dimension and comprises the derivative ofthe photosensitive material. The derivative of the photosensitivematerial comprises the first end group 22 and has hydrophilicity

The monolayer 14 may contain more than one first region 42 and/or morethan one second region 44; and typically, the patterned monolayer 14comprise a plurality of first regions 42 and a plurality of secondregions 44. Since the monolayer 14 includes a single layer of material,the first region 42 and the second region 44 laterally abut, and do notoverlie, each other.

A variety of photosensitive materials may be employed to form monolayer14. For instance, suitable photosensitive materials are disclosed incopending application Ser. No. 11/871,167, for “PhotosensitiveSelf-Assembled Monolayer For Selective Placement Of HydrophilicStructures,” the disclosure of which is herein incorporated by referencein its entirety.

As one example, a suitable photosensitive phosphonic acid may besynthesized in the following manner.

A mixture of methyl 4-bromomethylbenzoate and triethyl phosphate isformed and heated at an elevated temperature. For example, 10 grams ofmethyl 4-bromomethylbenzoate and 40 grams of triethyl phosphate may bemixed and heated at 120° C. for 4 hours. The solution is cooled andexcess triethyl phosphite may be evaporated under reduced pressure toform a light yellow oil, which is methyldiethylphosphonatomathyl-benzoate. Methyldiethyl-phosphonatomathylbenzoate in the form of the light yellow oilmay be used without farther purification in the next step.

Methyl diethylphosphonatomathylbenzoate is then dissolved in methanoland lithium hydroxide is added to the solution. For example, 10 grams ofmethyl diethylphosphonatomathyl-benzoate may be dissolved in 50 ml ofmethanol and 40 ml of 5% lithium hydroxide may be added. The mixture isthen stirred, for example, at room temperature for 5 hours. Methanol isthen evaporated under reduced pressure and the aqueous solution is thenmade acidic by addition of dilute hydrochloric acid. The precipitate isfiltered, washed with water, and then dried. Recrystallization fromwater produces white crystals of 4-diethylphosphonatomethyl benzoicacid.

4-diethylphosphonatomethyl benzoic acid is then added to anhydrousdichloromethane. For example, 5.0 grams of 4-diethylphosphonatomethylbenzoic acid may be added to 50 ml of anhydrous dichloromethane. Excessamount of oxalyl chloride is added to this solution. A trace amount ofN,N-dimethylformamide is then added to the solution. The mixture may bestirred, for example, under nitrogen for 4 hours. The solvent and theexcess oxalyl chloride is then evaporated under reduced pressure toproduce a colorless oil, which is 4-diethylphosphonato-methyl benzoicacid.

4-diethylphosphonato-methyl benzoic acid thus obtained in the form ofthe colorless oil is then dissolved in anhydrous dichloromethane. Thesolution is then added to a solution of 2-nitrobenzyl alcohol indichloromethane containing triethylamine. After stirring 4 hours at roomtemperature, the solution is washed with 5% sodium bicarbonate, dilutehydrochloric acid and brine successively, and then dried over anhydrousmagnesium and filtered. The solvent is then evaporated under reducedpressure and solid residue is crystallized from ethanol to give aphosphonate ester. The ester is dissolved in anhydrous dichloromethaneand treated with 4 equivalent of bromotrimethylsilane, and then stirredunder nitrogen for 5 hours. Methanol and a few drops of hydrochloricacid are added and stirring may be continued for an additional 1 hour.The precipitate is filtered and washed with ether, dried andcrystallized from ethanol to form photosensitive phosphonic acid aswhite crystals.

The above procedure may be performed with the replacement of the2-nitrobenzyl alcohol with 4-trifluoromethyl-2-nitrobenzyl alcohol. A4-substituted photosensitive phosphonic acid is obtained by thisprocess. Purification of this compound may be performed throughcrystallization from ethanol.

Further, the above procedure may be performed with the replacement ofthe 2-nitrobenzyl alcohol with 4-hexadecyloxy-2-nitrobenzyl alcohol.Another 4-substituted photosensitive phosphonic acid is obtained by thisprocess. Purification of this compound may be performed throughcrystallization from toluene-ethanol mixture.

In general, various alcohols may be employed to produce variousphotosensitive phosphonic acids.

With reference to FIG. 5, after the desired pattern of regions 42 and 44is formed on surface 16 of structure 10, an aqueous (or alcohol)solution solution of a dopant compound 52 (e.g. boric acid, organicboronic acid, phosphonic acids, etc.) is deposited on the semiconductorsurface by, for example, spin coating or doctor blading. These depositedmaterials will only attach on the hydrophilic regions 44.

Any suitable dopant solution may be used; and for instance, the solutionmay comprise a blend of first and second polar organic solvents having asilicon oxide-forming compound and a boron compound dissolved therein.The first solvent may have a boiling point of 50° C. to 150° C. andconstitute about 60% to 90% by weight of the total composition. Thesecond solvent may have a boiling point between 185° C. to 300° C. andconstitute about 0.5% to 30% of the composition. For example, thesolvent blend may include 2-ethoxyethanol (cellosolve), boiling at 135°C., and dimethylphthalate boiling at 282° C., in a ratio of about 13:1,respectively.

In one embodiment, the composition may also include boric acid or boronoxide (B₂O₃) as the dopant species, and a silicon oxide-forming compoundprepared by reacting tetraethylorthosilicate with acetic anhydride,which yields an equilibrium mixture of ethylacetate, triethoxysiliconacetate and diethoxysilicon diacetate.

Other solvents which may be substituted for ethoxyethanol include loweralcohols, such as ethanol; lower ketones, including acetone and methylethyl ketone; and alkyl ethers, such as ethyl ether and methyl ethylether, in addition to other alkoxyalcohols, such as methoxyethanol.

Other solvents that may be substituted for dimethylphthalate includeother aromatic esters, such as lower alkyl phthalates, i.e., methylethyl phthalate, diethyl phthalate and dipropyl phthalate; lower alkylisophthlates, such as diethylisophthilate; and salicylates, such asethyl salicylate, methyl salicylate and isoamyl salicylate.

The conductivity type-determining dopant for diffusion in silicon isgenerally selected from boron, phosphorous, and arsenic. Gold is also auseful dopant for lifetime control. These dopants may be added to thecompositions in the form of boron oxide, orthophosphoric acid,orthoarsenic acid, and gold chloride, respectively. Other dopant speciesare useful, with essentially equivalent results. Zinc chloride is asuitable source of zinc for diffusion in gallium arsenide.

The composition may include about 60% to 90% by weight solvent, and aratio of silicon atoms to dopant atoms of about 1.5:1 up to about 6:1,depending primarily upon the doping level required in the semiconductor.The molar ratio of acetic anhydride to tetraethylorthosilicate added maybe about 1.5:1 up to 3:1, and, as a more specific example, about 2.0:1up to 2.3:1.

Any suitable procedure may be employed to deposit the dopant solution onthe substrate surface 16, and for example, as mentioned above, spincoating or doctor blading may be used.

After deposition, the substrate 12 is then coated with a very thin layerof oxide, shown in FIG. 6 at 62 (e.g. aluminum oxide or hafnium oxide)by atomic layer deposition to cap the compounds, and then the substrateis annealed at high temperatures to diffuse the dopant atoms into thesilicon and to activate the dopant. Heating to diffusion temperatures ofabout 1100° C., for example, causes dopant to pass into thesemiconductor, as will be readily appreciated by one skilled in the art.Initial heating at a low temperature is optional.

While it is apparent that the invention herein disclosed is wellcalculated to fulfill objects discussed above, it will be appreciatedthat numerous modifications and embodiments may be devised by thoseskilled in the art, and it is intended that the appended claims coverall such modifications and embodiments as fall within the true spiritand scope of the present invention.

1. A method of fabricating a semiconductor device, comprising: providinga semiconductor substrate including an oxide surface; patterning saidsurface into hydrophobic and hydrophilic regions; depositing a compoundincluding a dopant on the substrate, wherein the dopant adheres to thehydrophilic region; and diffusing the dopant into the oxide surface ofthe substrate.
 2. The method according to claim 1, further comprisingcapping the dopant compound on the substrate.
 3. The method according toclaim 2, wherein the capping includes forming an oxide layer over theoxide surface of the substrate.
 4. The method according to claim 3,wherein the diffusing includes annealing the substrate after saidcapping to diffuse the dopant into the oxide surface of the substrate.5. The method according to claim 1, wherein the diffusing includesannealing the substrate to diffuse the dopant into the oxide surface ofthe substrate.
 6. The method according to claim 1, wherein the dopantdoes not adhere to the hydrophobic region.
 7. The method according toclaim 1, wherein said compound is an aqueous or alcohol solution.
 8. Themethod according to claim 1, wherein the patterning includes: applying amonolayer of a photosensitive material to said oxide surface; andexposing the monolayer to radiation to form the monolayer into a patternof said hydrophobic an hydrophilic regions, said regions including atleast one hydrophobic region and at least one hydrophilic region.
 9. Themethod according to claim 8, wherein: the monolayer includes a first,hydrophilic group attached to the oxide surface; the exposing includesexposing a portion of said first group; and the depositing includeadhering the dopant compound to said exposed portion of the first groupof the monolayer.
 10. The method according to claim 9, wherein themonolayer further includes a second, photosensitive group and a third,hydrophobic group; and the dopant compound does not adhere to saidthird, hydrophobic group of the monolayer.
 11. A method of fabricating asemiconductor device, comprising: providing a substrate having an oxidesurface; applying a monolayer of a photosensitive material to said oxidesurface; exposing the monolayer to radiation to form the monolayer intoa pattern of non-overlapping first and second regions, said firstregions being hydrophobic, and said second regions being hydrophilic;depositing a solution including dopants on the monolayer, after saidexposing, wherein the solution adheres to said first regions; removingthe solution from said second regions to form patterned dopant regionson said substrate; and diffusing the dopants from the solution into saidoxide surface.
 12. The method according to claim 11, further comprisingcapping the dopants on said substrate.
 13. The method according to claim12, wherein said Capping layer is an oxide layer.
 14. The methodaccording to claim 11, wherein the hydrophic regions of the monolayerare substantially free of the dopant compound.
 15. The method accordingto claim 14, wherein the diffusing includes annealing the substrate todiffuse the dopants into the oxide surface.
 16. A semiconductor devicecomprising: a substrate having an oxide surface; a monolayer of aphotosensitive material applied in said oxide surface, said monolayerincluding patterned hydrophobic and hydrophilic regions; a dopantcompound applied to said hydrophilic regions to form a dopant pattern onthe oxide surface.
 17. The semiconductor device according to claim 16,further comprising a capping layer over the dopant on the oxide surface.18. The semiconductor device according to claim 17, wherein the cappinglayer is an oxide layer.
 19. The semiconductor device according to claim16, wherein the hydrophobic regions of the monolayer are substantiallyfree of the dopant compound.
 20. The semiconductor device according toclaim 19, wherein the hydrophobic regions do not overlap the hydrophilicregions.
 21. A semiconductor structure comprising: a substrate having anoxide surface; a monolayer of a photosensitive material applied on saidoxide surface, said monolayer of photosensitive material including afirst portion having an outer, hydrophilic group and a second portionnot having said outer, hydrophilic group; a dopant compound applied tothe monolayer of the photosensitive material, wherein the dopantcompound attaches to the first portion of the photosensitive materialand does not attach to the second portion of the photosensitive materialto form a dopant pattern on the oxide surface.
 22. The semiconductorstructure according to claim 21, further comprising a capping layer overthe dopant on the oxide surface.
 23. The semiconductor structureaccording to claim 22, wherein the capping layer is an oxide layer. 24.The semiconductor structure according to claim 21, wherein thehydrophobic regions of the monolayer are substantially free of thedopant compound.
 25. The semiconductor device according to claim 24,wherein the hydrophobic regions do not overlap the hydrophilic regions.